Much deeper cutbacks on top of already curtailed spending are in store for the sea services if Congress can’t resolve looming budget crises, the top officers of the Navy and Marine Corps said Thursday.

Resulting trims in operations spending would force them to ground aircraft, temporarily close maintenance depots, and delay or cancel ship deployments – moves that would deeply affect San Diego and likely bankrupt some smaller defense businesses, according to the Commandant of the Marine Corps, Gen. James Amos, and the Chief of Naval Operations, Adm. Jonathan Greenert.

The cuts have a disproportionate effect on the Marine Corps, the smallest of the Pentagon services, because of its small budget, Amos said Thursday afternoon, at a conference sponsored by the U.S. Naval Institute and AFCEA, a trade group.

In addition to a spending cap at 2012 levels until Congress passes a new appropriations bill, the military is facing more than a trillion dollars in spending cuts over the next decade — if the automatic rollback in defense spending known as “sequestration” is triggered in March, on top of cuts the Pentagon previously instituted.

Amos put those numbers in perspective by noting that the entire Marine Corps budget for 2012 including money for the war in Afghanistan is about $30 billion. The cost of refurbishing or replacing all its equipment now in the war zone is about $3.2 billion.

“That’s the bill for the future readiness of the Marine Corps, and I am very concerned about that. That’s what keeps me up at night,” Amos said. “There is, in my sense, a lack of appreciation for the sheer magnitude of the cost.”

As it stands, the Corps has been supporting forces bound for combat by borrowing from operations and maintenance accounts. “They are going to get 100 percent of everything they need – people, equipment, training. We will mortgage everything for the forces that are in Afghanistan,” Amos said. “But that mortgage is being paid by home station readiness, home station equipment, equipment that is not going into depots, equipment that we are not buying to go into reset.”

The Marine Corps is considering shuttering maintenance depots for the third and fourth quarters if sequestration takes effect. That would force the Corps to ground some tactical aircraft next year, including F/A-18 fighter jets and EA-6B Prowlers whose service life has been stretched while the Corps waits for the F-35 Joint Strike Fighter jet under development.

“Sequestration is going to have a significant impact on all of us. We are going to have to go back in and figure out, OK, how are we going to be able to pay the bills? The operations and maintenance, which is sailing dates, it’s fuel, it’s flight hours, it’s wear and tear on equipment, it’s reset for equipment,” Amos said. “If sequestration happens, San Diego is going to feel it. So is Norfolk, Va., so is Camp Pendleton and Camp LeJeune.”

The Corps is also getting squeezed by maintenance cutbacks for Navy amphibious assault ships that deploy infantry and aircraft assigned to Marine Expeditionary Units.